We've been using Visual Studion 2005 Beta 2 since it came out. At that time, we decided to use the Team Test unit testing rather than NUnit. I've just read Jonathan Hodgson's post on pricing, which is rather disheartening. Unit testing is the only feature of the Team System version that we need, but the upgrade price is five times higher. In our case, unit testing is going to cost us over £1,100 per developer. It's good and everything, but no better than NUnit and TestDriven.NET, both of which are free (at the moment). It's going to be much cheaper to convert all our tests to NUnit than to upgrade to the Team System version.
I was speaking to Nick Page of Microsoft yesterday, and he said it's a common problem.